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Rings of ice and dust may encircle Pluto

By Maggie McKee

Pluto is too far from Earth for its surface to have been imaged in detail – this picture is an artist’s impression

(Image: NASA/JPL)

Faint, icy rings may periodically appear around Pluto when small space rocks smash into its two recently discovered moons, new research suggests. NASA’s New Horizons spacecraft, currently en route to the planet, will search for the rings – and may have to change course to avoid them.

Pluto is surrounded by a large moon, Charon, which at 1200 kilometres wide is about half the planet’s diameter. But in 2005, astronomers using the Hubble Space Telescope discovered Pluto is also orbited by two tiny moons – dubbed S/2005 P1 and S/2005 P2. These moons appear to be about 60 and 50 kilometres wide and orbit Pluto every 38 days and 25 days, respectively.

Now, researchers who discovered them say the moons are so puny that any dust kicked up by impacts from small rocks is likely to float away, rather than fall back onto the moons. It could then produce tenuous rings around Pluto that may survive for up to 100,000 years. A similar process is thought to produce some of Saturn’s famous rings and the faint rings around Jupiter.

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“If Pluto’s small moons generate debris rings from impacts on their surfaces, as we predict, it would open up a whole new class of study because it would constitute the first ring system seen around a solid body rather than a gas giant planet,” says team member Bill Merline of the Southwest Research Institute in Boulder, Colorado, US.

Such debris rings would have occurred sporadically over the age of the solar system, says team member Hal Weaver of Johns Hopkins University in Laurel, Maryland, US&colon; “It’s a random process.”

Ring of danger

Even though the rings around Pluto are thought to be faint and thin, they could potentially damage New Horizons, which is due to fly within 10,000 kilometres of the distant planet in 2015. “We are flying very fast and even small particles could be disastrous if they hit the spacecraft,” Weaver told New Scientist. “If we knew about any ring system, we would try to avoid flying through it.”

Such a change in trajectory could mean the spacecraft missing out on some of its science goals, which include flying into Pluto’s shadow and looking back at the planet to study its thin atmosphere. Weaver suspects such a drastic trajectory change will not be necessary, but adds very careful consideration is required “because of the high consequence of what could happen”.

The newly published research also confirms the small moons orbit Pluto on circular paths and in the same plane as Charon. That suggests all three moons coalesced from a disc of debris created after a large object smashed into Pluto early in the history of the solar system. They also suggest Pluto may harbour even smaller moons – less than 25 kilometres wide – that Hubble does not have the resolution to see.

The work suggests “tenuous arcs or rings may be the rule, rather than the exception, for Pluto and other multiple-bodied congregations” in the outer solar system, comments Richard Binzel of the Massachusetts Institute of Technology in Cambridge, US.

He adds that observations to date suggest 20% of all objects in the Kuiper Belt, a ring of icy objects beyond Neptune, contain at least one moon. “Even quadruple systems may become passé as investigations become increasingly percipient,” he writes in the journal Nature.